In the modern telecommunications world, voice communications continue to be a popular mode of communication, but new services like video telephony, high speed data and short message services continue to expand on existing services. The arrival of new telecommunications services generates new requirements for telecommunications networks. New telecommunications techniques (transfer modes) are required and offer possible advantages compared to existing techniques. Traditional transfer modes for wired communications are circuit switching, familiar in classical telephone services, and packet switching, familiar in telegraphy and modern short message service and data systems.
Asynchronous transfer mode (ATM) is a mode of fast packet switching which allows systems to operate at a much higher rate than traditional packet switching systems. Features which characterize ATM communications are: the ability for asynchronous operations between a sender clock and a receiver dock; transmission "cells" of pre-defined sizes; and addressing carried out in a fixed size header (that is not but time, frame position or other fixed characteristic). ATM communication is sometimes also referred to as asynchronous time division (ATD) communications.
Asynchronous transfer mode (ATM) is a mode of fast packet switching which facilitates switching systems that operate at a much higher rate than traditional packet switching systems. Features which characterize ATM communications are: the ability for asynchronous operations between a sender clock and a receiver clock; the concept of a "virtual connection" which is established for the lifetime of an information flow that comprises part or all of the communication; transmission "cells" of a fixed, standardized size; and connection identification carried in a fixed size header (that is not by time, frame position or other fixed characteristic). ATM communication is sometimes also referred to as asynchronous time division (ATD) communications. Other features of ATM communications are notions of a "service category", "traffic contract" and Quality of Service objectives that apply to the virtual connection. The expression "virtual connection" here is used to refer a virtual path and virtual circuit pair and "virtual connection identifier" means either a virtual path identifier (VPI) or a virtual circuit identifier (VCI) or both.
ATM communication has proven useful in high-value point-to-point land-line communication, for example, satellite links and undersea cables. ATM allows multiple simultaneous circuits, sometimes referred to as virtual circuits (VCs), to be established from end to end along the link.
European Patent No. EP0679042 of Roke Manor Research describes a mobile communications network with ATM as the transfer mode used in the switching infrastructure and describes steps to be taken in the mobile network switching infrastructure when a mobile terminal changes affiliation from one base station to another base station, as in a conventional handoff operation and when a mobile terminal communicates simultaneously through more than one base station. The transfer mode of the radio link is not described. International Patent Application No. WO94/28645 of The Trustees of Columbia University in the City of New York also addresses the use of ATM in a mobile communications system switching network and addresses distributed call set-up and rerouting in a mobile ATM based system with ATM switches.
A mobile communications network consists of a number of mobile end systems, a number of base stations, and a number of base station controllers, where the base stations and base station controllers are interconnected using an Asynchronous Transfer Mode (ATM) network. When a mobile end system moves from radio site (or "cell" or "zone") to another, it is necessary to execute a handoff between the corresponding base stations.
The standardized ATM architecture prohibits any ATM network (including a wireless ATM network) from misordering or duplicating ATM user data cells. In general, ATM networks should lose (i.e. by discarding) few, or preferably no, ATM user data cells at any time, including during handoff. Further, the ATM service architecture distinguishes between `real time` and `non-real time` service categories. In real time service categories, cell delay variation (CDV--the variability in the pattern of cell arrival events at the output of an ATM connection relative to the pattern of corresponding events observed at the input of the connection) is an element of quality of service. CDV is negotiated between the end systems (including mobile end systems) and the network(s). If a cell exceeds the agreed CDV, then it either is lost, or becomes useless to the end system when it is delivered; thus, a late cell is treated as if it were lost. Non-real time services are indifferent to CDV, but may be more sensitive to cell discard.
The arrangements described in the above prior art patent application are not optimal in their use of ATM resources in an access network, nor do those arrangements address communication using ATM as the transfer mode over-the-air.
International Patent Application No. WO94/32594 of NTT Mobile Communication Network, Inc. describes a cellular mobile radio communication system soft-handover scheme using code division multiple access (CDMA) where signals transmitted from different base stations are spread with different spread codes and simultaneously received at a mobile station with reception units in correspondence to different base stations. It is described how communication can take place in packets which include a call number, in case the mobile station deals with a plurality of calls, a sequence number and an identification number (ID) for the mobile station. It is explained how the same packet can be received at the mobile station from more than one base station or received at more than one base station from the same mobile station, to provide a reliable diversity handover scheme. The establishment of simultaneous communication through two base stations is described, without the completion of a handover process being described. It must be assumed that the completion of handover complies with pre-existing CDMA soft handover principals. The patent application also mentions that the packet communication scheme can be an ATM scheme.
Attention is turning to the use of ATM for the radio interface transfer mode of wireless communications. There is, for example, a need for wireless users to have access to wired ATM networks and existing ATM systems such as multi-media applications need a wireless platform providing multi-media support. It is also recognized that systems such as universal mobile telephone systems (UMTS) and existing wireless local area networks (LANs) cannot meet all future data user needs. Efforts to date have been in the use of ATM in the wireless extension of fixed infrastructure systems, such as local area networks (LANs) and integrated service data network (ISDN).
For private land mobile networks and cellular radio networks, circuit-switched frequency-division multiple access (FDMA) with or without time division multiple access (TDMA), as well as code division multiple access (CDMA) continue to be the available multiple access schemes for the radio interface. Each of these multiple access schemes has its advantages and disadvantages in different circumstances and the various schemes are generally incompatible with each other.
A mobile radio system is now envisaged using ATM as the transfer mode and using a novel multiple access scheme which has advantages over existing FDMA, TDMA and CDMA multiple access schemes. There is a need for a method of handover in such a novel system.
______________________________________ Glossary of Terms ______________________________________ ATM Asynchronous Transfer Mode BS Base station BSC Base station controller CDV Cell Delay Variation CLP Cell Loss Priority GFC Generic Flow Control HEC Header Error Control PTI Payload Type Identifier VPI Virtual Path Identifier VCI Virtual Circuit Identifier ______________________________________